pmtud package

internal/pmtud/message.go

@@ -0,0 +1,58 @@
+package pmtud
+
+import (
+	cryptorand "crypto/rand"
+	"encoding/binary"
+	"fmt"
+	"math/rand/v2"
+
+	"golang.org/x/net/icmp"
+	"golang.org/x/net/ipv4"
+	"golang.org/x/net/ipv6"
+)
+
+func buildMessageToSend(ipVersion string, mtu int) (id uint16, message *icmp.Message) {
+	var seed [32]byte
+	_, _ = cryptorand.Read(seed[:])
+	randomSource := rand.NewChaCha8(seed)
+
+	const uint16Bytes = 2
+	idBytes := make([]byte, uint16Bytes)
+	_, _ = randomSource.Read(idBytes)
+	id = binary.BigEndian.Uint16(idBytes)
+
+	var ipHeaderLength int
+	var icmpType icmp.Type
+	switch ipVersion {
+	case "v4":
+		ipHeaderLength = ipv4.HeaderLen
+		icmpType = ipv4.ICMPTypeEcho
+	case "v6":
+		ipHeaderLength = ipv6.HeaderLen
+		icmpType = ipv6.ICMPTypeEchoRequest
+	default:
+		panic(fmt.Sprintf("IP version %q not supported", ipVersion))
+	}
+	const pingHeaderLength = 0 +
+		1 + // type
+		1 + // code
+		2 + // checksum
+		2 + // identifier
+		2 // sequence number
+	pingBodyDataSize := mtu - ipHeaderLength - pingHeaderLength
+	messageBodyData := make([]byte, pingBodyDataSize)
+	_, _ = randomSource.Read(messageBodyData)
+
+	// See https://www.iana.org/assignments/icmp-parameters/icmp-parameters.xhtml#icmp-parameters-types
+	message = &icmp.Message{
+		Type:     icmpType, // echo request
+		Code:     0,        // no code
+		Checksum: 0,        // calculated at encoding (ipv4) or sending (ipv6)
+		Body: &icmp.Echo{
+			ID:   int(id),
+			Seq:  0, // only one packet
+			Data: messageBodyData,
+		},
+	}
+	return id, message
+}